Stem Cell Factor (SCF) Synergizes with Megakaryocyte Colony Stimulating Activity in Post-irradiated Aplastic Plasma in Stimulating Human Megakaryocytopoiesis

Plasma obtained from lethally irradiated animals contains a megakaryocyte (MK) growth factor which has recently been identified as the ligand for the c-mpl receptor and has been named thrombopoietin (TPO). We demonstrate that post-irradiation aplastic canine plasma (PICS-J) and plasma from a human subject (ML) who was accidentally exposed to lethal irradiation, contain high levels of this activity, which support both MK proliferation and maturation in a dose-dependent manner. These plasma were far more active in stimulating human MK colony formation than other types of thrombocytopenic plasma or a number of exogenously added human recombinant cytokines and their combinations. The addition of stem cell factor (SCF), which alone has a minimal stimulatory affect, to post lethal-irradiation plasma provided a synergistic stimulation of megakaryocytopoiesis both in colony assays and liquid cultures. In colony assays, the combination of SCF with PICS-J or ML almost doubled the number of burst forming units (BFU-MK) and provided a 1.5-fold increase in colony forming units (CFU-MK). A 1.6-fold increase in the number of CD34+ BM cell-derived MK colonies was also elicited. In liquid cultures, the presence of both SCF and PICS-J or ML induced the appearance of a high proportion of CD34+ (6.56% vs 0.6% control) and CD41+ (3.5% vs 1.2% control) cells after 3 days in culture. By day 10, 66.8 x 10(4) CD41+ cells and 29.8 x 10(4) CD34+ cells were derived from 2 x 10(6) BMMC originally seeded. We propose that these unique plasma, which do not contain elevated level of IL-6, IL-3, GM-CSF, IL-1 beta, erythropoietin or SCF, probably contain high levels of TPO. The addition of SCF to the post-irradiation plasma provides a synergistic stimulation of megakaryocytopoiesis which may become relevant for future clinical application.